Police have searched homes in the family's neighborhood, dug through a nearby landfill and searched waterways and drainage systems in Tucson. More than 1,000 tips have poured in regarding the six-year-old's disappearance, police said.

Isabel's parents made several public, emotional pleas for the safe return of their daughter in the days after her disappearance.

"Just please, please, to the person or persons who have Isabel, tell us what you want. We will do anything for her. We're looking for you, Isa," Sergio Celis said tearfully.

The search has also extended into Mexico, where authorities in the state of Sonora have been circulating photos of Isabel.